Electronic heating apparatus including microwave coupling structure and filters therefor



4 Sheetg-Shet l O 8 8 56 5 6 4 2 I 3 3 2 2 4 2 l 2 2 I 1 r L m w 2 2 5 6v v 7 f4. 2 Q wlwdgf L, i w 2 l m 4 I I 1U w 2 w L. H. FITZMAYER ET ALSTRUCTURE AND FILTERS THEREFOR I m 23 v I! u n, 30 370 30/ 320 3/ FE TTELECTRONIC HEATING APPARATUS INCLUDING MICROWAVE COUPLING Oct. 27, 1970Filed Dec. 51, I 1968 Oct. 27, 1970 F|TZMAYER ET Al. 3,536,878

ELECTRONIC HEATING APPARATUS INCLUDING MICROWAVE COUPLING STRUCTUREAND.FILTERs THEREFOR Filed Dec. 31, 1968 4 Sheets-Sheet 2 FIG. 2

v 320 327 v 345 330 370 30/ 372 v 37/ 326 322 31/ 3/6 Oct. 27, 1970 H.FITZMAYER ETAL 3,536,878

ELECTRONIC HEATING APPARATUS INCLUDING MICROWAVE COUPLING STRUCTURE ANDFILTERS THEREFOR Filed Dec. 31, 1968 4-She ets-Sheet 5 wvm :n 9% W N 8m1 won cow own wkm in in I mmm J M 3m 1 kmn mmm V 9m mom son Q9 vmw 9N 5A www 7 wo 6w mat mON 0 E W kON IN wQN ER ET AL CLUDING MI 3,536,878CROWAVE COUPLING 4 Sheets-Sheet 4 t- 1970 L. H. FITZMAY ELECTRONICHEATING APPARATUS IN STRUCTURE AND FILTERS THEREF Filed Dec. 31, 1968United States Patent Int. Cl. HOSb 9/06 US. Cl. 219-1055 23 ClaimsABSTRACT OF THE DISCLOSURE There is disclosed an electronic ovencomprising a generator of microwave energy of a predetermined frequencycoupled to a cooking cavity and to a source of DC and 60 cycle operatingpotentials by a coupling structure, the coupling structure including aplurality of capacitive bypass filters and resonant cavity rejectionfilters for preventing the propagation of microwave energy to the sourceand preventing propagation of harmonics of the predetermined frequencyto the cooking cavity, two of the resonant cavity filters, respectivelyresonant at the second and fourth harmonics of the predeterminedfrequency, being disposed one within the other.

This invention is concerned with an improved microwave couplingstructure and an improved transmission line forming a part of anelectronic oven.

' More particularly, this invention concerns improved microwavefiltering elements for the coupling structure and the transmission line.

It is a general object of this invention to provide an electronicheating apparatus including structure defining a cooking cavity, agenerator for generating electromagnetic wave energy of a predeterminedultra-high-frequency and having a pair of output terminals, atransmission line coupling the generator to the cooking cavity andincluding a hollow outer conductor coupled to one terminal of thegenerator and an inner conductor coupled to the other terminal of thegenerator, a plurality of filter sections in the inner conductor forattenuating the second and third and fourth harmonics of thepredetermined frequency and a filter section in the transmission lineproviding a low impedance bypass path for the second and higherharmonics of the predetermined frequency, whereby during thetransmission of electromagnetic wave energy along the transmission linethe harmonics of the predetermined frequency are highly attenuated bythe filter sections while the predetermined frequency is propagated tothe cooking cavity substantially without attenuation.

In connection with the foregoing object, it is another object of thisinvention to provide an electronic heating apparatus of the type setforth wherein each of the filter sections in the inner conductorcomprises structure defining a cavity, the cavities having effectiveelectrical lengths respectively equal to one-quarter of the wavelengthof the second and third and fourth harmonics of the predeterminedfrequency and being respectively resonant at the second and third andfourth harmonics of the predetermined frequency to effect highattenuation thereof.

In connection with the foregoing objects, it is yet another object ofthis invention to provide an electronic heating apparatus of the typeset forth and including a filter section in the inner conductorcomprising structure defining an inner cavity and an outer cavitydisposed about the inner cavity, the filter section structure having alength equal to about one-quarter of the wavelength of the secondharmonic of the predetermined frequency, the inner cavity having aneffective electrical length equal to "ice one-quarter of the wavelengthof the fourth harmonic of the predetermined frequency and being resonantat the fourth harmonic of the predetermined frequency, the outer cavityhaving an effective electrical length equal to one-quarter of thewavelength of the second harmonic of the predetermined frequency andbeing resonant at the second harmonic of the predetermined frequency,the filter section presenting a high impedance to the transmission ofboth the second and fourth harmonics of the predetermined frequencyalong the inner conductor while having a length equal to aboutone-quarter of the wavelength of the second harmonic of thepredetermined frequency.

Another object of this invention is to provide an electronic heatingapparatus of the type set forth wherein the transmission line includes afirst cup-like filter member including a cylindrical sidewallsurrounding the adjacent portion of the inner conductor andconcentrictherewith and spaced therefrom and an end wall closing thesidewall at one end thereof and connected to the inner conductor, thefirst filter member cooperating with the inner conductor to define afirst cavity having an effective electrical length equal to one-quarterof the wavelength of the fourth harmonic of the predetermined frequencyand being resonant at the fourth harmonic of the predeterminedfrequency, and a second cup-like filter member including a cylindricalside wall surrounding the cylindrical side wall of the first filtermember and concentric therewith and spaced therefrom and an end wallclosing the side wall of the second filter member at one end thereof andconnected to the inner conductor, the second filter member cooperatingwith the first filter member and with the inner conductor to define asecond cavity having an effective electrical length equal to onequarterof the wavelength of the second harmonic of the predetermined frequencyand being resonant at the second harmonic of the predeterminedfrequency, the first filter member being disposed within the secondfilter member and the second filter member having a length equal toabout one-quarter of the wavelength of the second harmonic of thepredetermined frequency While the filter members present a highimpedance to the transmission of both the second and fourth harmonics ofthe predetermined frequency along the inner conductor, whereby duringthe transmission of electromagnetic wave energy along the transmissionline the second and fourth harmonics of the predetermined frequency arehighly attenuated by the filter members While the predeterminedfrequency is propagated to the cooking cavity substantially withoutattenuation.

In connection with the foregoing object, it is another object of thisinvention to provide electronic heating apparatus of the type set forth,and further including a third cup-like filter member including acylindrical side wall surrounding the adjacent portion of the innerconductor and concentric therewith and spaced therefrom and an end wallclosing the side wall at one end thereof and connected to the innerconductor, the third filter member cooperating with the inner conductorto define a cavity, having an effective electrical length equal toone-quarter of the wavelength of the third harmonic of the predeterminedfrequency and being resonant at the third harmonic of the predeterminedfrequency for highly attenuating the transmission along the transmissionline of electromagnetic wave energy at the third harmonic of thepredetermined frequency.

It is another object of this invention to provide a transmission linefor interconnecting a generator for generating electromagnetic waveenergy of a predetermined ultra-high frequency and a load wherein thegene erator has a pair of output terminals, the transmission linecomprising a hollow outer conductor coupled to one terminal of thegenerator, an inner conductor disposed within the outer conductorconcentric therewith and coupled to the other terminal of the generator,a first filter section in the inner conductor and comprising structuredefining a first cavity having an eleffective electrical length equal toone-quarter of the wavelength of the second harmonic of thepredetermined frequency and being resonant at the second harmonic of thepredetermined freqeuncy for effecting high attenuation thereof, and asecond filter section in the inner conductor comprising structuredefining a second cavity having an effective electrical length equal toone-quarter of the wavelength of the fourth harmonic of thepredetermined frequency and being resonant at the fourth harmonic of thepredetermined frequency for effecting high attenuation thereof, wherebyduring the transmission of electromagnetic wave energy along thetransmission line the second and fourth harmonics of the predeterminedfrequency are highly attenuated by the filter section while thepredetermined frequency is propagated to the load substantially withoutattenuation.

In connection with the foregoing object, it is another object of thisinvention to provide a transmission line of the type set forth, andfurther including a third filter section in the inner conductorcomprising structure defining a third filter cavity having an effectiveelectrical length equal to one-quarter of the wavelength of the fourthharmonic of the predetermined frequency and being resonant at the fourthharmonic of the predetermined frequency for effecting high attenuationthereof.

In connection with the foregoing objects, it is still another object ofthis invention to provide a transmission line of the type set forth andincluding a filter section in the inner conductor comprising structuredefining an inner cavity and an outer cavity disposed about the innercavity, the filter section structure having a length equal to aboutone-quarter of the wavelength of the second harmonic of thepredetermined frequency, the inner cavity having an effective electricallength equal to one-quarter of the wavelength of the fourth harmonic ofthe predetermined frequency and being resonant at the fourth harmonic ofthe predetermined frequency, the outer cavity having an effectiveelectrical length equal to one quarter of the wavelength of the secondharmonic of the predetermined frequency and being resonant at the secondharmonic of the predetermined frequency, the filter section presenting ahigh impedance to the transmission of both the second and fourthharmonics of the predetermined frequency along the inner conductor whilehaving a length equal to about one-quarter of the wavelength of thesecond harmonic of the predetermined frequency.

Another object of this invention is to provide a transmission line ofthe type set forth and including a first cup-like filter memberincluding a cylindrical side wall surrounding the adjacent portion ofthe inner conductor and concentric therewith and spaced therefrom and anend wall closing the side wall at one end thereof and connected to theinner conductor, the first filter member cooperating with the innerconductor to define a first cavity having an effective electrical lengthequal to onequarter of the wavelength of the fourth harmonic of thepredetermined frequency and being resonant at the fourth harmonic of thepredetermined frequency, and a second cup-like filter member including acylindrical side wall surrounding the cylindrical side will of the firstfilter member and concentric therewith and spaced therefrom and an endwall closing the side Wall of the second filter member at one endthereof and connected to the inner conductor, the second filter membercooperating with the first filter member and with the inner conductor todefine a second cavity having an effective electrical length equal toone-quarter of the wavelength of the second harmonic of thepredetermined frequency and being resonant at the second harmonic of thepredetermined frequency,

the first filter member being disposed within the second filter memberand the second filter member comprising a filter section having a lengthequal to about one-quarter of the wavelength of the second harmonic ofthe predetermined frequency while the filter members present a highimpedance to the transmission of both the second and fourth harmonics ofthe predetermined frequency along the inner conductor, whereby duringthe transmission of electromagnetic wave energy along the transmissionline the second and fourth harmonic of the predetermined frequency arehighly attenuated by the filter members while the predeterminedfrequency is propagated to the load substantially without attenuation.

In connection with the foregoing object, it is another object of thisinvention to provide a transmission line of the type set forth andfurther including a third cup-like filter member including a cylindricalside wall surrounding the adjacent portion of the inner conductor andconcentric therewith and spaced therefrom and an end wall closing theside wall at one end thereof and connected to the inner conductor, thethird filter member cooperating with the inner conductor to define athird cavity having an effective electrical length equal to one-quarterof the wavelength of the third harmonic of the predetermined frequencyand being resonant at the third harmonic of the predetermined frequencyfor effecting high attenuation thereof.

In connection with the foregoing objects, another object of theinvention is to provide a transmission line of the type set forth andfurther including a filter section comprising a low impedance bypasspath between the inner and outer conductors for the second and higherharmonics of the predetermined frequency to effect high attenuationthereof.

It is another object of this invention to provide a coupling structurefor interconnecting a generator for generating RF energy of apredetermined ultra-high RF frequency and a source of DC operatingpotentials having at least one terminal and a source of low frequency ACenergy having one terminal, wherein the generator has an annular outerRF terminal and an inner RF terminal, the coupling structure comprisinga hollow outer conductor coupled to the outer RF terminal of thegenerator and extending outwardly therefrom and terminating in an outerend, an inner conductor coupled to the inner RF terminal of thegenerator and extending outwardly therefrom and terminating in an outerend adjacent to the outer end of the outer conductor, a pair of RFoutput terminals respectively coupled to the outer conductor and theinner conductor intermediate the generator RF terminals and the outerends, means coupling the low frequency AC source terminal and the DCsource terminal to the outer end of the inner conductor, and a pluralityof filter elements disposed in the conductors between the RF outputterminals and the outer ends of the conductors and providing lowimepedance bypass paths for the second and third harmonics of thepredetermined frequency and including structure defining cavitiesrespectively resonant at the predetermined frequency and the fourthharmonic thereof for presenting a high series impedance to thetransmission of the predetermined frequency and the fourth harmonicthereof along the conductors to the low frequency AC source terminal andthe DC source terminal, whereby propagation of RF energy at thepredetermined frequency and the second and third and fourth harmonicsthereof from the generator to the outer ends of the conductors is highlyattenuated without interfering with the transmission of the lowfrequency AC and the DC operating potentials between the sources thereofand the generator.

In connection with the foregoing object, another object of thisinvention is to provide a coupling structure of the type set forthincluding first and second cup-like filter members each including acylindrical side wall surrounding the adjacent portion of the innerconductor and con centric therewith and spaced therefrom and an end wallclosing the side wall at one end thereof and connected to the innerconductor, the filter members being telescopically received in andinsulated from the outer conductor, and wherein the inner and outerconductors are spaced apart and cooperate to form therebetween a lowimpedance bypass path at the second harmonic of the predeterminedfrequency, and the cylindrical side wall of the first filter membercooperates with the outer conductor to form therebetween a low impedancebypass path at the third harmonic of the predetermined frequency, andthe first filter member cooperates with the inner conductor to define afirst cavity having an effective electrical length equal to one-quarterof the wavelength of the predetermined frequency and being resonant atthe predetermined frequency, and the second filter member cooperateswith the inner conductor to define a second cavity having an effectiveelectrical length equal to onequarter of the wavelength of the fourthharmonic of the predetermined frequency and being resonant at the fourthharmonic of the predetermined frequency.

Another object of the invention is to provide a coupling structure forinterconnecting a generator for generating RF energy of a predeterminedultra-high RF frequency and a source of DC operating potentials havingat least one terminal and a source of low frequency AC operatingpotential having at least one terminal and a load for the RF energy,wherein the generator has an annular outer RF terminal and an inner RFterminal, the coupling structure comprising a first hollow outerconductor coupled to the outer RF terminal of the generator andextending outwardly therefrom and terminating in an outer end, a firstinner conductor disposed in the first outer conductor and coupled to theinner RF terminal of the generator and extending outwardly therefrom andterminating at an outer end adjacent to the outer end of the first outerconductor, means coupling the DC source terminal and the low frequencyAC source terminal to the outer end of the first inner conductor, asecond hollow outer conductor coupled to the first outer conductorintermediate the generator RF terminals and the outer ends and coupledto the load, a second inner conductor disposed in the second outerconductor and coupled to the first inner conductor intermediate thegenerator RF terminals and the outer ends and coupled to the load,filter elements disposed in the'first outer conductor between thecoupling to the second outer conductor and the outer ends of the firstconductors and providing a low impedance bypass path for the secondharmonic of the predetermined frequency and including structure definingtwo resonant cavities respectively resonant at the predeterminedfrequency and the fourth harmonic thereof for presenting a high seriesimpedance to the transmission of the predetermined frequency and thefourth harmonic thereof along the first conductors to the DC sourceterminal and the low frequency AC source terminal, whereby propagationof RF energy at the predetermined frequency and the second and fourthharmonics thereof from the generator to the outer ends of the firstconductors is highly attenuated without interfering with thetransmission of the DC and low frequency AC operating potentials betweenthe sources thereof and the generator, and filter sections in the secondouter conductor comprising structure defining two cavities respectivelyresonant at the second and fourth harmonics of the predeterminedfrequency to present a high impedance to the transmission of the secondand fourth harmonics of the predetermined frequency along the secondconductors, whereby propagation of RF energy at the second and fourthharmonics of the predetermined frequency along the second conductors ishighly attenuated without interfering with the transmission of microwaveenergy at the predetermined frequency to the load.

Further features of the electronic heating apparatus pertain to theparticular arrangement of the parts of the transmission line andcoupling assembly forming a part thereof, whereby the above-outlined andadditional operating features thereof are attained.

The invention, both as to its organization and its method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the following specification taken inconnection with the accompanying drawings in which:

FIG. 1 is a side elevational view with certain portions broken away ofan electronic heating apparatus made in accordance with and embodyingthe principles of the present invention;

FIG. 2 is a fragmentary view on an enlarged scale in. vertical sectionof a crossed-field discharge device forming a part of the generator forthe apparatus of FIG. 1;

FIG. 3 is a top plan view on a reduced scale of the generator of FIG. 2including a portion of a transmission line for coupling the generator tothe cooking cavity in the apparatus of FIG. 1;

FIG. 4 is a fragmentary view in vertical section through thetransmission line forming a part of the apparatus of FIG. 1 andinterconnecting the device of FIG. 2 to the cooking cavity of theapparatus;

the line 88 of FIG. 5;

FIG. 9 is an enlarged view in vertical section along the line 99 of FIG.6; and

FIG. 10 is an enlarged view in vertical section along the line 10-40 ofFIG. 6.

Referring now to FIG. 1 of the drawings, the electronic heatingapparatus 10 there illustrated, and embodying the features of thepresent invention, is in the form of a combination electric andelectronic range that is especially designed for home use. Moreparticularly, the range 10 comprises an upstanding substantiallybox-like casing 11 formed of steel and including a pair of side walls12-, a rear wall 13 having a removable closure member or panel 15disposed therein, and a top wall 14 and a bottom wall 16, the removablepanel 15 being removably held in place by a plurality of screws 27. Thecasing 11 houses in the upper central portion thereof a metal liner 20defining a heating or cooking cavity 21 therein, the metal liner 20preferably being formed of steel, and essentially comprising a box-likestructure provided with a top wall 22, a bottom wall 23, a rear Wall 24,and a pair of opposed side walls 25; whereby the liner 20 is providedwith an upstanding front opening into the heating cavity 21 definedtherein. Further, the casing 11 is provided with a front door 28arranged in a front opening formed therein and cooperating with thefront opening provided in the liner 20, the front door 28 being mountedadjacent to the lower end thereof upon associated hinge structure 29;whereby the front door 28 is movable between a substantially verticalclosed position and a substantially horizontal open position withrespect to the front opening provided in the liner 20'.

An electric heating unit '36 is arranged in the upper portion of theheating cavity 21 adjacent to and below the top wall 22, and an electricheating unit 37 is arranged in the lower portion of the heating cavity21 adjacent to and above the bottom wall 23; which electric heatingunits 36 and 37 are utilized in the carrying out of conventional bakingand broiling cooking operations in the heating cavity 21, as explainedmore fully hereinafter. Also, a temperature sensing bulb 38 is arrangedin a pocket provided in one of the side walls 25; which temperaturesensing bulb 38 forms a part of an oven switch and temperaturecontroller and is utilized in carrying out the previously mentionedconventional baking and broiling operations in the heating cavity 21.The side walls of the liner 20 further carry thereon a plurality ofshelf supports 26 for the supporting of shelves (not shown) that in turnsupport items to be cooked within the heating cavity 21. There also isprovided below the front door 28 a lower front panel 19 that closes afront opening in the lower portion of the casing 11, the bottom wall 16further being provided with a reticulated member or screen 17 and thelower portion of the rear wall 13 being provided with a reticulatedmember or screen 18, the screens 17 and 18 permitting the passage of airthrough the lower portion of the casing 11 to cool the electronicapparatus therein as will be described more fully hereinafter.

Disposed below the liner 20 and spaced therefrom is a I generallyhorizontally arranged lower bafile 20 extending laterally across thecasing 11 between the side walls 12 thereof and extending rearwardlyfrom the front of the casing 11 to a point spaced forwardly of the rearwall 13, the baffle in cooperation with the casing 11 defining a bottommachinery compartment in the lower portion of the casing 11, the lowerfront panel 19 being removable to provide access to the bottom machinerycompartment 35.

Mounted on the underside of the baflie 30 is an electric 'motor 31having an output drive shaft 32 including a reduced diameter portion 33extending upwardly into the liner 20 and supporting thereon a turntable34 formed of expanded sheet metal and arranged in the lower portion ofthe heating cavity 21. The turntable 34 is mounted for rotation and uponoperation of the motor 31 is adapted to support food to be heated orcooked in the electronic operation that is carried out in the heatingcavity 21, as explained more fully hereinafter. The motor 31 has anassociated gear train (not shown) that reduces the speed of the shaft 33to approximately 6 r.p.m.

A rear baffie is provided to the rear of the liner 20 and extends acrossthe casing 11 between the side walls 12 thereof, the baflie 40 includinga main wall 41 carrying centrally thereof a rearwardly offset wall 42that is in general horizontal alignment with the rear of the liner 20and spaced rearwardly therefrom. Disposed in the lower portion of themain wall 41 is an opening around which is disposed a flange 43connecting with an air duct 44 that communicates with the screen 1 8 inthe rear wall 13 of the casing 11. It further will be noted that thebottom bafiie 30 carries on the rear thereof an upwardly and rear'wardlyextending baffle section 39 that extends toward the offset wall 42 butis spaced therefrom, the baffies 30 and 40 being formed of metal, suchas steel, whereby the spacing between the baifie section 39 and the rearbaffle 40 minimizes the conduction of heat therebetween during theoperation of the range 10. The rear baffle 40 cooperates with the casing11 to provide a rear machinery compartment 45, the rear machinerycompartment 45 being disposed behind the liner 20 and access theretobeing provided through the removable panel 15 that covers the opening inthe rear wall 13 described above.

There is arranged to the right (as viewed in FIG. 1) of thebottom'machinery compartment 35 a generator for supplying ultra-highfrequency electromagnetic wave energy for electronic cooking within thecooking cavity 21, the generator 50 including a crossed-field electronicdischarge device 100 of the construction and arrangement disclosed inthe copending application of James E. Staats, Ser. No. 559,267, filedJune 21, 1966, now US. Pat. No. 3,458,755. Referring to FIGS, 1 and 2,it will be seen that the device 100' is disposed within a box-likestructure or casing 101 that extends completely about the device 100 butis open on two opposed sides thereof, the sides disposed to the left andright in FIG. 2,

the device and the associated parts therefor being mounted within andelectrically connected to the casing 101. As will be explained morefully hereinafter, high operating DC potentials are present on thecasing 101, whereby it is desirable electrically to isolate and shieldthe casing 101, and to this end a second box-1ike structure 102 has beenprovided that surrounds the casing 101 and is also provided on anopposed pair of sides thereof with openings in alignment with theopposed open sides of the casing 101, all to accommodate the passage ofair through the casing 101 and the box-like structure 102 to cool thedevice 100 and the associated parts housed therein. However, in order toprevent .contact of the user with the casing 101 and the device 100disposed therein, reticulated metal covers 103 are provided for coveringthe openings in the opposed sides of the box-like structure 102, thecovers 103 permitting passage of air therethrough and thus through thecasing 101 and the structure 102 while preventing contact of a user withthe casing 101 and the parts disposed therein that carry the highoperating DC potentials. Further, the casing 101 is mounted uponinsulators 104 carried by the structure 102, thereby electrically toinsulate the casing 101 from the structure 102.

As viewed in FIG. 1, the structure 102 and the casing 101 disposedtherein are disposed to the rear of the bottom apparatus 35, and aredisposed to the right within the bottom apparatus compartment 35 whenthe range 10 is viewed from the front. In order to provide cooling airfor passage through the reticulated'covers 103 and across the device100, there has been provided an open ended housing 105 disposed to theleft in FIG. 1 or in front of the structure 102 and housing therein atleast a part of a voltage doubler and rectifier circuit 130 thatsupplies DC operating potentials to the device 100 and also housestherein a fan 106 powered by a motor 55 within the housing 105, an airduct 107 being provided about the fan 106 to direct air therefrom intothe structure 102 and into the casing 101 and about the device 100. Morespecifically, the fan 106 operates to draw air through the screen 17 atthe bottom of the apparatus compartment 35, the air being formed into astream by the housing 105 and passed over the rectifier 130, the motor55 and through the air duct 107 into the structure 102; the air streamwithin the structure 102 passes into the casing 101 and about the device100 and cooling fins 118 disposed thereon and passes therefrom and intothe air duct 44 to be discharged through the screen 18 in the rear wall13 of the casing 11.

In accordance with the present invention, the air stream generated bythe fan 106 is used to cool all of the various parts of the generator50, and specifically the crossed-field discharge device 100 and thevoltage doubler and rectifier circuit 130 associated therewith. To thisend the housing 105 and the air duct 107 and the air duct 44 have beenprovided so as to concentrate the air stream upon the parts noted, allwhile attempting to deflect the air stream away from the baffles 30 and40. The bafiies 30 and 40 further protect the liner 20 and the cookingcavity 21 disposed therein from the air stream thus generated so as tomaintain more uniform cooking conditions within the cooking cavity 21and thus to improve the cooking therein.

Referring now particularly to FIG. 2 the generator 50 there illustratedwill be described in greater detail. An electron discharge device 100 iscontained within a substantially cylindrical metal envelope 111 andincludes anode and cathode structure (not shown). Surrounding theenvelope 111 and connected thereto is a plurality of cooling fins 118for dissipating heat from the device 100 as explained above. In order toestablish a unidirectional magnetic field within the device 100 there isprovided a composite magnetic field winding 120a and 12011 respectivelydisposed at the lower and upper ends of the device 100 and connected inseries relation by a conductor 123. A DC operating potential from avoltage doubler and rectifier 130 is applied to the winding 12% by aconductor 122, and from the winding 120a to the device 100 by aconductor 124 which is connected to one of the cooling fins 118 as at125. Further details of the construction and operation of the device 100are disclosed in the previously mentioned Staats application Ser. No.559,267, the disclosure of which is incorporated herein by reference.

The generator 50 is arranged to be advantageously operated in connectionwith suitable control and power supply apparatus, details of theconstruction and operation of which are disclosed in the copending U.S.applications of James E. Staats, Ser. No. 656,977, filed June 12, 1967now US. Pat. No. 3,421,115, Ser. No. 569,006, filed June 27, 1966-, nowabandoned and Ser. No. 181,144, filed Mar. 20, 1962, the disclosures ofall of which applications are incorporated herein by reference.

The device 100 is operative to supply ultra-high frequency energy ofabout 915 mc., with a power output at the output terminals in thegeneral range 50 to 800 watts. The device is arranged to supply the .RFpower for cooking and to this end a lower transmission line 300 extendsfrom the device 100 and is coupled to an upper transmissiOn line 200which extends to the cooking cavity 21, the transmission lines 200 and300 being of the coaxial type including an inner conductor and anenclosing outer conductor electrically insulated therefrom. Both of theoutput terminals of the device 100' are at a substantial voltage DC withrespect to ground potential, so that the output terminals areelectrically insulated from ground potential, as well as from eachother. One of the output terminals is coupled by a capacitor 317 to theadjacent end of the outer output conductor, and the other outputterminal is coupled by a capacitor 318 to the adjacent end of the inneroutput conductor, the remote end of the inner conductor projecting anantenna 201 into the oven cavity 21; the remote end of the outerconductor is electrically connected to the metal liner 20. Thus the RFpower produced by operation of the device 100 is radiated from theantenna 201 into the cooking cavity .21, so as to produce cookingeffects upon food arranged therein, all in a conventional manner.

An important feature of the present invention resides in theconstruction and connection of the transmission lines 200 and 300interconnecting the crossed-field discharge device 100 and the liner 20,a detailed description of the transmission line 200 and 300 being nowpresented with particular reference to FIGS. 3 through of the drawings.The cross-field discharge device 1100 is provided with a magnet yoke 126at the upper end thereof which connects to the anode of the device 100(not shown) and forms an outer conductor and an output terminal for thedevice 100, the lower end of the mangetic yoke 126 being moreparticularly connected to the anode of the device 100 and the upper endextending upwardly through the field winding 12017 and being connectedto a magnet flange 127. The lower end of thedevice 100 is likewiseprovided with a magnet yoke 128 having the upper end thereof connectedto the anode of the device 100 and the other end extending downwardlythrough the field winding 120a and being connected at the lower end toa. magnet flange 129. The cathode of the device 100 (not shown) hasconnected thereto a stud 351 forming a part of the upper couplingstructure 350, the stud 351 and the magnet yoke 128 forming a coaxialoutput connection for the device 100.

Referring specifically to FIG. 4, it will be seen that the output fromthe other end of the upper transmission line 200 is coupled to theantenna 201 disposed in the heating cavity 21, the antenna 201 beingessentially a rod having a threaded stud 202 at one end thereof, theantenna 201 extending into the cavity 21 a short distance below the topwall 22 and essentially midway between the side walls 25, see FIG. 1.The stud 202 extends into a complementarily threaded opening at the endof a rod 217 forming an inner conductor that extends into the cavity 21,the

conductor 217 being held in position by a bracket 204 and a cooperatingelectrical insulator 208. More specifically, the bracket 204 includes anannular mounting flange 205 which is suitably secured as by welding tothe rear wall 24 and of the liner 20 and disposed about an openingtherein, the flange 205 in turn carrying a forwardly directed annularflange 206 terminating in an inturned holding flange 207 disposedagainst the forward surface of the insulator 208.

A top section 210 of the transmission line 200 is formed by a first tube211 forming the outer conductor therefor and the inner conductor 217.The forward edge of the tube 211 has an outwardly directed flange 212thereon which is suitably secured as by welding on the rear side of therear wall 24 and surrounding the opening therein and assisting inholding the insulator 208 in the operative position in cooperation withthe bracket 204.

The rear end of the tube 211 is received in a T 220, the T 220 includinga body 221 and a pair of arms 222 and 224 and a leg or arm 226 allprovided with a seat or shoulder 223, 225 and 227, respectivelyinternally thereof. The rear end of the tube 211 is disposed within thearm 222 and in telescopic relationship therewith and has the rearrnostend thereof spaced slightly forwardly with respect to the shoulder 223,an insulator 218 being disposed between the tube 211 and the shoulderand held in position thereby, the insulator 218 having a central openingtherein receiving the inner conductor 217 therethrough. Preferably theouter end of the arm 222 has longitudinally extending slots (not shown)formed therein and receives therearound a clamp 215 having associatedtherewith a screw 216 (see FIG. 1) for tightening the clamp 215 aboutthe outer end of the arm 222, thus releasably to clamp the arm 222 aboutthe rear end of the tube 211 to hold it in the operative position. Itfurther is pointed out that the arm 222 extends through an opening 46 inthe baflle wall 42, whereby the major portion of the T 220 is disposedto the rear of the baffle wall 42 and thus in the rear machinerycompartment 45, whereby the T 220 is accessible for service andmaintenance through the removable panel 15 (see FIG. 1 also). The otherarm 224 carries on the outer end thereof a dust cover 228 that serves toclose the same to prevent the entry of dust, dirt, water and the likeinto the interior of the T 220.

A rear section 230 for the transmission line 200 is provided by an outertube 231 and an inner conductor 232 disposed within the outer tube 231.The upper end of the tube 231 extends into the lower arm or leg 226 ofthe T 220 and is spaced from the shoulder 227 thereof, an insulator 224being disposed between the upper end of the tube 231 and the shoulder227 and held in position therebetween. The insulator 234 has an openingtherethrough to receive the upper end of the inner conductor 232, theinner conductor 232 having an upper flattened end 233 having an openingtherein receiving therethrough a screw 219 engaging in a complementarilythreaded opening in the rear end of the rod 217, whereby to interconnectthe rear end of the inner conductor 217 and the upper end of the innerconductor 232. There further is provided around the lower portion of theleg 226 one of the clamps 215, the lower end of the leg 226 preferablybeing slotted (not shown) whereby tightening of the screw 216 associatedwith the clamp 215 serves to tighten the leg 226 about the upper end ofthe tube 231 releasably to hold the parts in the assembled telescopingpositions.

The lower end of the rear transmission line section 230 is received in aT 240, the T 240 including a body 241 having a pair of arms 242 and 244and a leg or arm 246 each provided with a cooperating internal seat orshoulder 243, 245 and 247, respectively. More particularly, the leg 246is positioned upwardly and receives telescopically therein the lower endof the tube 231, the lower end of the tube 231 being spaced upwardlywith respect to the seat 247 to receive therebetween an insulator 236 tomount the same within the T 240, the insulator 236 having an openingtherethrough receiving the lower end of the inner conductor 232therethrough. The outer end of the leg 246 is preferably slotted (notshown) and receives therearound one of the clamps 215, whereby by meansof the associated screw 216 the clamp 215 can be tightened about theouter end of the leg 246 to clamp it against the lower end of the tube231 releasably to hold the parts in the assembled position. The arm 244preferably carries thereon a dust cover 248 which cover serves toprevent the entry of dust, dirt, water and other debris into theinterior of the T 240 and the transmission lines 200 and 300.

There is operatively associated with the other arm 242 of the T 240 arear section 310 of the lower transmission line 300, the rear section310 including a tube 311 as the outer conductor and a tube 314 as theinner conductor spaced therefrom and electrically insulated therefrom.The tube 311 further has a portion 312 of reduced diameter that extendsinto and is telescopically associated with the arm 242 of the T 240, theinner end of the tube 311 being spaced from the shoulder 243 andreceiving therebetween an insulator 238 that is held in positionthereby. One of the clamps 215 is disposed about the outer end of thearm 242 which is preferably slotted (not shown), whereby by means of theassociated screw 216 the clamp 215 can be tightened to press the arm 242about the adjacent end 312 of the tube 311 releasably to hold the partsin the assembled position. The insulator 238 has an opening thereinreceiving therethrough the inner conductor 314, the rear end of theinner conductor 314 carrying a plug 315 secured thereto and having athreaded opening therein, the lower end of the rear inner conductor 232having a flattened portion as at 235 having an opening therein receivinga screw 237 that threadedly engages the opening in the plug 315, thus tointerconnect the lower end of the inner conductor 232 and the rear endof the inner conductor 314. The forward end (the end disposed to theleft of FIG. 4) of the tube 311 is outwardly flared as at 313, and theforward end of the inner conductor 314 is outwardly flared as at 316,all to permit ready assembly with other portions of the transmissionline 300 as will be described more fully hereinafter.

The rear lower section 310 cooperates with and is telescopicallyassociated with a front lower transmission line section 320, the frontlower section 320 including a tube 321 serving as the outer conductorand an annular inner conductor 322 disposed within the tube 321 andspaced therefrom and electrically insulated therefrom. The tube 321 hasan external diameter slightly less than the internal diameter of themain portion of the tube 311 whereby the rear end of the tube 321 istelescopically received therein, a layer of insulation 317 beingdisposed therebetween to provide a capacitive coupling at certain R-Ffrequencies. The annular inner conductor 322 has received in an opening323 therethrough and threadedly engaged therewith a probe 330, the probe330 having an external diameter slightly less than the internal diameterof the inner conductor 3 14 whereby the rear end of the probe .330 canbe disposed therein, a layer of insulation 318 being providedtherebetween and forming therewith a capacitive coupling at certain RFfrequencies. An annular insulator 319 is disposed about the forward endof the layer of insulation 318 abutting the flared end 316 of the innerconductor 314 positively to position the parts one with the other.

The rearward end of the inner conductor 322 has a hollow portiongenerally designated 325 including a relatively thin cylindrical sidewall 327 and an annular end wall 328. The annular side wall 327 isdisposed in surrounding relationship With the adjacent portion of theinner conductor 330 concentric therewith and spaced therefrom and thehollow rearward end 325 of the inner conductor 322 cooperates with theadjacent portion of the inner conductor 330 to define an annular cavity329 therebetween. Disposed within and surrounded by the hollow end 325of the inner conductor 322 is a cup-like electrically conductive member335 having a cylindrical side Wall 336 disposed in surroundingrelationship with the adjacent portion of the inner conductor 330concentric therewith and spaced therefrom and being closed at theforward end thereof by an end wall 337. The end wall 337 of the cup-likemember 335 is disposed rearwardly of the end wall 328 of the hollow end325 of the inner conductor 322 and has an internally threaded opening338 therethrough along the longitudinal axis thereof. The innerconductor 330 is received through the opening 338 and is threadedlyengaged thereat with the cup-like member 335. Disposed about the innerconductor 330 and spaced rearwardly from the inner conductor 3212 andthe cup-like member 335 is another cup-like electrically conductivemember 340 having a cylindrical side wall 341 disposed in surroundingrelationship with the adjacent portion of the inner conductor 330concentric therewith and spaced therefrom, the side Wall 341 beingclosed at the rearward end thereof by an annular end wall 342. The endwall 342 has an internally threaded opening 343 therethrough along thelongitudinal axis thereof through which is received the inner conductor330, the inner conductor 330 being threadedly engaged thereat with thecuplike member 340.

Disposed about the inner conductor 330 and separating the open ends ofthe inner conductor 322 and the cup-like members 335 and 340 from oneanother is an annular insulating spacer 345. The spacer 345 has on theouter surface thereof a plurality of shoulders 346, 347 and 348 forrespectively supporting the open ends of the cup-like member 335, theinner conductor 322 and the cup-like member 340. More particularly, therearward end of the cylindrical side wall 336 of the cup-like member 335overlies the shoulder 346 and is thereby supported and spaced from theinner conductor 330; the rearward end of the cylindrical side wall 327of the inner conductor 322 similarly overlies the shoulder .347 and issupported thereby and spaced from the side wall 336 of the cup-likemember 335; the forward end of the cylindrical side wall 341 of thecup-like member 340 overlies and is supported by the shoulder 34 8 andis thereby spaced from the inner conductor 330. The shoulders 346, 347and 348 are arranged stepwise and are dimensioned so as to properlyspace the open ends of the inner conductor 322 and the cup-like members335 and 340 from one another to provide the proper reactive impedancesat this juncture as will be more fully described hereinafter.

The forward end of the rear lower transmission line section 320 iscoupled to the crossed-field discharge device by means of a T 370, the T370 including a body 371 having a pair of arms 372 and 374 and a leg orarm 376 each provided with a cooperating internal seat or shoulder 373,375 and 377, respectively. More particularly, the leg 376 is positioneddownwardly and is telescopically received in the upper end of the magnetyoke 126 for coupling thereto, the T 370 forming a part of the uppercoupling structure 350. Also forming a part of the upper couplingstructure 350 is a bullet 352 receiving into the lower end thereof thestud 351 that is coupled to the cathode of the device 100, the bullet352 having a shoulder 353 thereon that cooperates with the shoulder 377to hold in operative position an insulator 378 that serves to center thebullet 352 with respect to the leg 376. The upper end of the bullet 352carries an internally threaded eye 354 through which is threadedlyinserted a stud 324, one end of the stud 324 being threadedly receivedin a complementarily threaded opening in the inner conductor 322,thereby to support the forward end of the inner conductor 322, therealso being provided therearound a supporting insulator 326. The forwardend of the tube 321 is telescopically received within the leg 372 and isspaced a short distance away from the shoulder 373, thereby to clamp theinsulator 326 in the operative position therebetween. Preferably theforward end of the tube 321 is fixedly secured as by soldering to thearm 372.

A decoupling structure is carried by the other arm 374 of the T 370, andmore particularly comprises an inner conductor 355 having a rearward endconnected to the forward end of the stud 324 by means of a threadedconnection therebetween, the inner conductor 355 having a reducedforward portion 356. Fixedly mounted on the arm 374 and telescopicallyreceived therein is an outer conductor 357 within which is disposed aninsulating sleeve 359. The outer conductor 357 and the sleeve 359 bothextend outwardly beyond the outer end of the inner conductor 355. Acylindrical inner conductor 363 having an external diametersubstantially equal to the external diameter of the reduced forwardportion 356 of the inner conductor 355, is disposed forwardly of theinner conductor 355 along the longitudinal axis thereof. The rearwardend of the inner conductor 363 has a reduced diameter externallythreaded portion 364 which is threadedly engaged with a complementarythreaded opening 362 in the outer end of the reduced forward portion 356of the inner conductor 355. Disposed within the insulating sleeve 359 isa cup-like inner conductor 360 surrounding the reduced portion 356 ofthe inner conductor 355 and having an outer end closed by an end wall361 which abuts the reduced diameter portion 356 of the inner conductor355 at the outer end thereof, the threaded portion 364 of the innerconductor 363 passing through an opening in the wall 361. Anothercup-like conductor 365 is disposed in surrounding relationship with theadjacent portion of the inner conductor 363, the cup-like conductor 365being telescopically received Within the outer ends of the sleeve 359and the outer conductor 357 and having the outer end thereof closed byan end wall 366, the end wall 366 having an opening therein throughwhich is received a screw threadedly engaged with a complementarilythreaded Opening in the outer end of the inner conductor 363 fixedly tointerconnect the parts and to provide an input terminal 368.

In operation the inner conductors 322 and 330 and the cup-like members335 and 340 and the insulating spacer 345 cooperate to define aplurality of resonant cavities for filtering predetermined harmonics ofthe operating frequency of the device 100. More particularly, thecylindrical side wall 327 of the outer end 325 of the inner conductor322 has a length corresponding to about one-quarter of the wavelength ofthe second harmonic of the operating frequency of the device 100. Theside wall 327 and the end wall 328 are so dimensioned that the cavity329 defined thereby has an effective electrical length equal toone-quarter of the wavelength of the second harmonic of the operatingfrequency of the device 100 and is resonant at this second harmonic.Similarly, the cup-like member 335 is so dimensioned that the cavity 339defined thereby in cooperation with the inner conductor 330 has aneffective electrical length equal to onequarter of the wavelength of thefourth harmonic of the operating frequency of the device 100 and isresonant at this fourth harmonic. In like manner, the cup-like member340 is of such dimensions that the cavity 344 defined thereby incooperation with the inner conductor 330 has an effective electricallength equal to one-quarter of the wavelength of the third harmonic ofthe operating frequency of the device 100 and is resonant at this thirdharmonic. The three resonant cavities thus provided re spectivelypresent a high series impedance to the transmission of the second andthird and fourth harmonics of the operating frequency of the device 100along the transmission line 300 thereby to provide high attenuation ofthese harmonics and to minimize the propagation thereof to the cookingcavity 21. Furthermore, the inner conductor 314 overlaps the innerconductor 330 a distance equivalent to about one-quarter of thewavelength of the frequency of operation of the device 100 and the outerconductors 311 and 321 overlap a distance equal to about one-quarter ofthe Wavelength of the frequency of operation of the device 100, therebyto provide a half-wave coaxial transmission line section at the secondharmonic of the frequency of operation of the device to provide a highimpedance path between the inner conductors 330 and 314 and the outerconductors 321 and 311 of the transmission line 300 for the second andhigher even harmonics of the frequency of operation of the device 100.

In actual construction, the above-described resonant cavity filters forthe second and third and fourth harmonies of the frequency of operationof the device 100 and the high impedance filter for the second andhigher even harmonics of the frequency of operation of the device 100are all contained in a portion of the transmission line 300 having anoverall length of about 6 /2 inches, Which at the frequency of operationof the device 100 (915 mHz.) represents about one-half wavelength.Accordingly, advantageous compaction of a large number of filters into arelatively small space has been achieved.

It is noted that the transmission of energy at the frequency ofoperation of the device 100 to the cooking cavity 21 along thetransmission line 300 is not substantially impeded by theabove-described harmonic filters. The various parts of the forwardsection 320 of the transmission line 300 have been so arranged and areof such dimensions that any capacitive reactance at 915 mHz. at thejunction of the cup-like member 340, the inner conductor 314 and theinsulator 319 is substantially canceled by inductive reactance at 915mHZ. at the junction of the inner conductor 322, the cup-like members335 and 340 and the insulator 345.

Because of the effective filtration of the second, third and fourthharmonics of the operating frequency of the device 100 in thetransmission line 300, it is necessary that these harmonics, as Well asthe frequency of operation itself be effectively filtered in thecoupling structure described above to prevent their propagation to theEdison supply network and the voltage doubler and rectifier circuit 130.To this end, the cup-like inner conductor 360 cooperates with the innerconductor 355 to define a cavity having an effective electrical lengthequal to one-quarter of the wavelength of the frequency of operation ofthe device 100 and being resonant thereat to provide a high seriesimpedance to the transmission of this frequency along the conductors 355and 357. The conductors 355 and 357 cooperate with each other to providea capacitive low impedance bypass path therebetween for the secondharmonic of the frequency of operation of the device 100. Similarly, thecup-like inner conductor 360 cooperates 'with the outer conductor 357 toprovide therebetween a low impedance bypass path at the third harmonicof the frequency of operation of the device 100. Finally, the cup-likeconductor 365 cooperates with the inner conductor 363 to define a cavityhaving an effective electrical length equal to one-quarter of thewavelength of the fourth harmonic of the frequency of operation of thedevice 100 and being resonant thereat to provide a high series impedanceto the transmission thereof along the conductors 363 and 357.

In effect, the inner conductor 355, the outer conductor 357 and thecup-like conductors 360 and 365 are so arranged and dimensioned thatthere is provided along the inner conductor 355 capacitive loading onthe second harmonic of the frequenecy of operation of the device 100.There is also provided at the inner end of the inner conductor 355 anopen circuit reflection at the frequency of operation of the device 100between the inner conductor 355 and the outer conductor 357; there isprovided at the inner end of the cup-like conductor 360 a short circuitreflection at the third harmonic of the frequency of operation of thedevice 100 between the conductors 360 and 357; and there is provided atthe inner end of the cup-like conductor 365 an open circuit reflectionat the fourth harmonic of the frequency of operation of the device 100between the conductors 365 and 363. Accordingly, the above-described RFfilters provide 15 a high attenuation for the frequency of operation ofthe device 100 and for the second and third and fourth harmonies thereofto minimize the propagation of RF energy to the Edison supply networkand the voltage doubler and rectifier circuit 130.

In the transmission line 300, the bullet 352, the T 370, the innerconductor 322 and the tube 321 are shaped and arranged to provide aquarter wave transformer section at the frequency of operation of thedevice 100. More particularly, the shouldered portions of the bullet 352and the conductor 322, the inner conductor 330 and the cup-like member335 have dimensions such that the impedance of the device 100 is matchedto the impedance of the transmission lines 200 and 300 that is in turnmatched to the impedance of the heating cavity 21. Likewise, the bullet352, the inner conductor 355, the outer conductor 357 and the innerconductor 360 are all shaped and arranged to provide a quarter wavetransformer section that assists in decoupling RF energy from the inputterminal 368 to prevent the propagation of RF energy into the powersupply. It is noted that the stepped configuration of the innerconductor 355 permits a shorter mechanical connection while maintainingan electrical characteristic equivalent to one-quarter wavelength of theoperating frequency of the device 100.

Because the potentials for operating the device 100 are derived from thevoltage doubler and rectifier circuit 130, neither the stud 351 formingthe inner conductor or any of the parts such as the yoke 126 and the T370 forming the outer conductor of the coupling structure 350 can begrounded. However, it is highly desirable to ground the portion of thetransmission line 200 disposed to the rear of the rear baffle 40, and tothis end the capacitive coupling at 317 and 318 has been providedbetween the rear lower transmission line portion 310 and the front lowertransmission line portion 320. Furthermore, the outer conductors 311 and321 overlap a distance equivalent to one-quarter wavelength at thefrequency of operation of the device 100 and the inner conductor 314overlaps the probe 330 a distance equivalent to one-quarter wave lengthof the frequency of operation of the device 100, thereby to provide afilter for the second and higher harmonies as well as providing a DCinsulation between the parts named. Accordingly, the outer conductor 311can be grounded as on the casing 11 and the baffle member 40, thereby topresent only grounded parts to workmen gaining access to the rearmachinery compartment 45 through the removable panel 15.

It is noted that the front panel 19 of the electronic heating apparatusis preferably removable. Due to the telescoping arrangement of the lowertransmission line portions 310 and 320, the device 100 and the frontportion 320 of the transmission line 300 may be easily disengaged fromthe rear portion 310 of the transmission line 300 and removed from thelower machinery compartment 35 through the front end thereof. Thus, theelectron discharge device is readily accessible from the front of therange for easy servicing.

It further is pointed out that the T 220, the entire rear transmissionline section 230, the T 240 and the rear lower transmission line section310 form a removable transmission line assembly that can be bodily movedrearwardly through the opening provided by the removable panel formaintenance and repair of the parts. Such movement of the transmissionline assembly rearwardly is accomplished by simply loosening the clamp215, about the leg 222 which frees the T 220 from the tube 211 andremoving the screw 219 to free the inner conductor 232 from the innerconductor 217. Due to the telescoping arrangement of the lowertransmission line portions 310 and 320, there is no need to remove ordisconnect any parts other than by the relative sliding movement of theportions 310 and 320 with respect to each other. Reassembly of the partsis facilitated by the flared ends 313 and 316 on the outer conductor 311and the inner conductor 314 for re-engagement over the conductors 321and 330, respectively. It further is necessary to hold the removabletransmission line assembly in the assembled position, and to this end aspring 249 under tension has been provided interconnecting the T 240 andthe bafiie wall 42, thus continually to urge the removable transmissionline assembly into the assembled operative position.

Another important feature of the transmission lines 200' and 300 residesin the fact that the tubes 211, 231, 311, 321 and 357 can all be formedessentially of standard tubing shaped as required and cut to length, thetubing preferably being formed of copper, brass or other goodelectrically conductive metal. The inner conductors 232 and 314 also canbe formed essentially of standard tubing shaped as required and cut tolength, this tubing also preferably being formed of copper, brass or thelike. The Ts 220, 240 and 370 are also of standard configuration and areall identical one to the other, the T' preferably being formed ofcopper, brass or other material having good electrical conductivity.Finally, the insulators 218, 234, 236, 238, 317-319, 326, 359 and 378are preferably all formed of a polytetrafiuoroethylene resin such asthat sold under the trademark Teflon.

A lower coupling structure 390 cooperates with the magnet yoke 128serving as an outer conductor of a transmission line and a tube 391connected at the upper end to one end of the heater of the device (notshown) and serving as an inner conductor of a coaxial transmission line,the lower end of the tube 391 carrying an insert 392 therein andreceiving the screw 396 serving as an input terminal. Disposed Withinand essentially lining the magnet yoke 128 is a sleeve 393 ofelectrically insulating material, an inner conductor 394 being disposedagainst the sleeve 393 and telescopically overlapping a portion of theyoke 128 and having the outer end thereof closed by an end wall 395, theend wall 395 having an opening therethrough receiving the shank of thescrew 396 that engages in a complementarily threaded opening in theinsert 392. The yoke 128, the tube 391, the insulating sleeve 393, theinner conductor 394 and the end wall 395 cooperate to provide a parallelresonant circuit including a reactive impedance and a capacitiveimpedance, the structure comprising a high impedance to RF energy toprevent propagation thereof onto the conductor 155. More specifically,the distance between the lower adjacent end of the anode of the device100 and the inner surface of the end wall 395 is equivalent to a quarterWavelength at the operating frequency of the device 100, and the yoke128 and the inner conductor 394 telescopically overlap a distanceequivalent to one-eighth wavelength at the operating frequency of thedevice 100.

From the above it will be seen that the liner 20 is effectively isolatedfrom the bottom machinery compartment 35 and the rear machinerycompartment 45 by the bafile members 30 and 40, respectively, thereby toprovide a more uniform distribution of heat within the liner 20 and thusto permit good cooking therein. The entire generator 50 including thecrossed-field discharge device 100 and the voltage doubler and rectifiercircuit 130 therefor are housed within the bottom machinery compartment35 which provides a protecting housing therefor. The fan 106 serves tocool all of the electrical components of the generator 50 by drawing airinwardly through the screen 17 into the bottom machinery compartment 35and across the crossed-field discharge device 100 and outwardly throughthe screen 18. The stream of air thus created is effectively preventedfrom coming into contact with the liner 20 due to the presence of thebaffie members 30 and 40. Also, the improved coupler structure andtransmission line 200 has been provided, the major portion of which canbe readily removed from the assembled relation with the liner 20 and thedevice 100 for repair and services purposes through the removable panel15, and can thereafter be readily reassembled therewith.

Further, it can be seen that there has been provided in the forwardsection 320 of the transmission line 300 a 1 7 plurality of filters forthe second and third and fourth and higher harmonics of the frequency ofoperation of the device 100, all of which filters have been disposedwithin a section of transmission line having a length equal to onlyabout one-half of the wavelength of the frequency of operation of thedevice 100, all without substantially impeding the propagation of energyat the frequency of operation of the device 100 to the cooking cavity21. Also, RF filters for filtering the frequency of operation of thedevice 100 and the second and third and fourth harmonics thereof havebeen provided in the coupling structure between the device 100 and theEdison supply network and the voltage doubler and rectifier circuit 130to provide high attenuation of RF energy while not interfering with thetransmission of the DC and low frequency AC operation potentials fromthe Edison supply network and the voltage doubler and rectifier circuit130 to the device 100.

From the above it wil be seen that there has been provided an improvedelectronic heating apparatus 10 and microwave coupling structure 350-390and improved transmission lines 200 and 300 therefor and improvedharmonic filters in the transmission line 300 which fulfill all of theobjects and advantages set forth above.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications can be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, a transmission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, a first filter section in said inner conductor andcomprising structure defining a first cavity having an effectiveelectrical length equal to one-quarter of the wavelength of the secondharmonic of the predetermined frequency and being resonant at the secondharmonic of the predetermined frequency to effect high attenuationthereof, a second filter section in said inner conductor and comprisingstructure defining a second cavity having an effective electrical lengthequal to one-quarter of the wavelength of the fourth harmonic of thepredetermined frequency and being resonant at the fourth harmonic of thepredetermined frequency to effect high attenuation thereof, wherebyduring the transmission of electromagnetic wave energy along saidtransmission line the second and fourth harmonics of the predeterminedfrequency are highly attenuated by said filter sections while thepredetermined frequency is propagated to said cooking cavitysubstantially without attenuation.

2. The electronic heating apparatus set forth in claim 1, and furtherincluding a third filter section in said transmission line including ahigh impedance path in said inner and outer conductors for the secondand higher even harmonics of the predetermined frequency to effect highattenuation thereof.

3. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, a transmission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, a first filter section in said inner conductor andcomprising structure defining a first cavity having an effectiveelectrical length equal to one-quarter of the wavelength of the secondharmonic of the predetermined frequency and being resonant at the secondharmonic of the predetermined frequency to effect high attenuationthereof, a second filter section in said inner conductor and comprisingstructure defining a second cavity having an effective electrical lengthequal to one-quarter of the wavelength of the third harmonic of thepredetermined frequency and being resonant at the third harmonic of thepredetermined frequency to effect high attenuation thereof, and a thirdfilter section in said inner conductor and comprising structure defininga third cavity having an effective electrical length equal to onequarterof the Wavelength of the fourth harmonic of the predetermined frequencyand being resonant at the fourth harmonic of the predetermined frequencyto effect high attenuation thereof, whereby during the transmission ofelectromagnetic wave energy along said transmission line the second andthird, and fourth harmonics of the predetermined frequency are highlyattenuated by said filter sections while the predetermined frequency ispropagated to said cooking cavity substantially without attenuation.

4. The electronic heating apparatus set forth in claim 3, and furtherincluding a fourth filter section in said transmission line including ahigh impedance path in said inner and outer conductors for the secondand higher even harmonics of the predetermined frequency to effect highattenuation thereof.

5. The electronic heating apparatus set forth in claim 4, wherein theportion of said transmission line including said first and second andthird and fourth filter sections has a length corresponding to aboutone-half of the wavelength of the predetermined frequency.

6. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic wave energy of apredetermined ultra-high frequency having a pair of output terminals, atransmission line coupling said generator to said cooking cavity andincluding a hollow outer conductor coupled to one terminal of saidgenerator and an inner conductor coupled to the other terminal of saidgenerator, and a filter section in said inner conductor and comprisingstructure defining an inner cavity and an outer cavity disposed aboutsaid inner cavity, said filter section structure having a length equalto about one-quarter of the wavelength of the second harmonic of thepredetermined frequency, said inner cavity having an effectiveelectrical length equal to one-quarter of the wavelength of the fourthharmonic of the predetermined frequency and being resonant at the fourthharmonic of the predetermined frequency, said outer cavity having aneffective electrical length equal to one-quarter of the wavelength ofthe second harmonic of the predetermined frequency and being resonant atthe second harmonic of the predetermined frequency, said filter sectionpresenting a high impedance to the transmission of both the second andfourth harmonics of the predetermined frequency along said innerconductor while having a length equal to about onequarter of thewavelength of the second harmonic of the predetermined frequency,whereby during the transmission of electromagnetic wave energy alongsaid transmission line the second and fourth harmonics of thepredetermined frequency are highly attenuated by said filter sectionWhile the predetermined frequency is propagated to said cooking cavitysubstantially without attenuation.

7. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic Wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, a trans mission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, a first filter section in said inner conductor andcomprising structure defining an inner cavity and an outer cavitydisposed about said inner cavity, said filter section structure having alength equal to about one-quarter of the wavelength of the secondharmonic of the predetermined frequency, said inner cavity having aneffective electrical length equal to one-quarter of the wavelength ofthe fourth harmonic of the predetermined frequency and being resonant atthe fourth harmonic of the predeter mined frequency, said outer cavityhaving an effective electrical length equal to one-quarter of thewavelength of the second harmonic of the predetermined frequency andbeing resonant at the second harmonic of the predetermined frequency,and a second filter section in said inner conductor and comprisingstructure defining a cavity having an effective electrical length equalto onequarter of the wavelength of the third harmonic of thepredetermined frequency and being resonant at the third harmonic of thepredetermined frequency, said first and second filter section'spresenting a high impedance to the transmission of the second and thirdand fourth harmonies of the predetermined frequency along said innerconductor, whereby during the transmission of electromagnetic waveenergy along said transmission line the second and third and fourthharmonics of the predetermined frequency are highly attenuated by saidfilter sections while the predetermined frequency is propagated to saidcooking cavity substantially without attenuation.

8. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, a transmission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, a first cup-like filter member including a cylindricalside wall surrounding the adjacent portion of said inner conductor andconcentric therewith and spaced therefrom and an end wall closing saidside wall at one end thereof and connected to said inner conductor, saidfirst filter member cooperating with said inner conductor to define afirst cavity having an effective electrical length equal to one-quarterof the wavelength of the fourth harmonic of the predetermined frequencyand being resonant at the fourth harmonic of the predeterminedfrequency, and a second cup-like filter member including a cylindricalside wall surrounding the cylindrical side wall of said first filtermember and concentric therewith and spaced therefrom and an end wallclosing said side wall of said second filter member at one end thereofand connected to said inner conductor, said second filter membercooperating with said first filter member and with said inner conductorto define a second cavity having an effective electrical length equal toone-quarter of the wavelength of the second harmonic of thepredetermined frequency and being resonant at the second harmonic of thepredetermined frequency, said first filter member being disposed withinsaid second filter member and said second filter member having a lengthequal to about one-quarter of the wavelength of the second harmonic ofthe predetermined frequency while said filter members present a highimpedance to the transmission of both the second and fourth harmonics ofthe predetermined frequency along said inner conductor, whereby duringthe transmission of electromagnetic wave energy along said transmissionline the second and fourth harmonics of the predetermined frequency arehighly attenuated by said filter members while the predeterminedfrequency is propagated to said cooking cavity substantially withoutattenuation.

9. The electronic heating apparatus set forth in claim 8, wherein theend walls of said first and second cup-like filter members are bothdisposed toward the same end of said transmission line.

10. The electronic heating apparatus set forth in claim 9, wherein theend walls of said first and second cup-like filter members are bothdisposed toward said generator.

11. The electronic heating apparatus set forth in claim 8, wherein theend walls of said first and second cup-like filter members each has aninternally threaded opening therethrough, said inner conductor beingdisposed through said internally threaded openings and threadedlyengaged thereat with said first and second cup=like filter members.

12. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, a transmission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, a first cuplike filter member including a cylindricalside wall surrounding the adjacent portion of said inner conductor andconcentric therewith and spaced therefrom and an end wall closing saidside wall at one end thereof and connected to said inner conductor, saidfirst filter member cooperating with said inner conductor to define afirst cavity having an effective electrical length equal to one-quarterof the wavelength of the fourth harmonic of the predetermined frequencyand being resonant at the fourth harmonic of the predeterminedfrequency, a second cup-like filter member including a cylindrical sidewall surrounding the cylindrical side wall of said first filter memberand concentric therewith and spaced therefrom and an end wall closingsaid side wall of said second filter member at one end thereof andconnected to said inner conductor, said second filter member cooperatingwith said first filter member and with said inner conductor to define asecond cavity having an effective electrical length equal to one-quarterof the wavelength of the second harmonic of the predetermined frequencyand being resonant at the second harmonic of the predeterminedfrequency, and a third cup-like filter member including a cylindricalside wall surrounding the adjacent portion of said inner conductor andconcentric therewith and spaced therefrom and an end wall closing saidside wall at one end thereof and connected to said inner conductor, saidthird filter member cooperating with said inner conductor to define acavity having an effective electrical length equal to one-quarter of thewavelength of the third harmonic of the predetermined frequency andbeing resonant at the third harmonic of the predetermined frequency,whereby during the transmission of electromagnetic wave energy alongsaid transmission line the second and third and fourth harmonics of thepredetermined frequency are highly attenuated by said filter memberswhile the predetermined frequency is propagated to said cooking cavitysubstantially without attenuation.

13. The electronic heating apparatus set forth in claim 12, wherein theend walls of said first and second cup-like filter members are bothdisposed toward one end of said transmission line and the end wall ofsaid third cup-like filter member is disposed toward the other end ofsaid transmission line.

14. The electronic heating apparatus set forth in claim 13, wherein theend walls of said first and second cup-like filter members are disposedtoward said generator and the end wall of said third cup-like filtermember is disposed toward said load.

15. The electronic heating apparatus set forth in claim 13, and furtherincluding an annular insulating spacer disposed around said innerconductor and separating said first and second cup-like filter membersfrom said third cup-like filter member, said insulating spacer havingthree pairs of shoulders thereon respectively abutting the ends of saidfirst and second and third cup-like filter members opposite to the endwalls thereof.

16. The electronic heating apparatus set forth in claim 12, wherein theend walls of said first and second and third cup-like filter memberseach has an internally threaded opening therethrough, said innerconductor being disposed through said internally threaded openings andthreadedly engaged thereat with said first and second and third cup-likefilter members.

17. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electro magnetic wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, a trans mission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, a first cup-like filter member including a cylindricalside wall surrounding the adjacent portion of said inner conductor andconcentric therewith and spaced therefrom and an end wall closing saidside wall at one end thereof and connected to said inner conductor, saidfirst filter member cooperating with said inner conductor to define afirst cavity having an effective electrical length equal to one-quarterof the wavelength of the fourth harmonic of the predetermined frequencyand being resonant at the fourth harmonic of the predeterminedfrequency, a second cup-like filter member including a cylindrical sidewall surrounding the cylindrical side wall of said first filter memberand concentric therewith and spaced therefrom and an end wall closingsaid side wall of said second filter member at one end thereof andconnected to said inner conductor, said second filter member cooperatingwith said first filter member and with said inner conductor to define asecond cavity having an effective electrical length equal to onequarterof the wavelength of the second harmonic of the predetermined frequencyand being resonant at the second harmonic of the predeterminedfrequency, a third cup-like filter member including a cylindrical sidewall surrounding the adjacent portion of said inner conductor andconcentric therewith and spaced therefrom and an end wall closing saidside Wall at one end thereof and connected to said inner conductor, saidthird filter member cooperating with said inner conductor to define athird cavity having an effective electrical length equal to one-quarterof the wavelength of the third harmonic of the predetermined frequencyand being resonant at the third harmonic of the predetermined frequency,and a filter section in said transmission line including a low impedancebypass path between said inner and outer conductors for the second andhigher harmonics of the predetermined frequency to effect highattenuation thereof, whereby during the transmission of electromagneticwave energy along said transmission line the second and third and fourthand higher harmonics of the predetermined frequency are highlyattenuated by said filter members and said filter section while thepredetermined frequency is propagated to said cooking cavitysubstantially without attenuation.

18. The electronic heating apparatus set forth in claim 17, wherein saidinner and outer conductors each comprise a pair of segments insulatedfrom each other and telescopically overlapping each other a distanceequal to about one-quarter of the wavelength of the predeterminedfrequency thereby providing a high impedance path in said inner andouter conductors for the second and higher even harmonics of thepredetermined frequency.

19. A coupling structure for interconnecting a generator for generatingRF energy of a predetermined ultrahigh RF frequency and a source of DCoperating potentials having at least one terminal and a source of lowfrequency AC energy having at least one terminal, wherein the generatorhas an annular outer RF terminal and an inner RF terminal, said couplingstructure comprising a hollow outer conductor coupled to the outer RFterminal of the generator and extending outwardly therefrom andterminating in an outer end, an inner conductor disposed in said outerconductor and coupled to'the inner RF terminal of the generator andextending outwardly therefrom and terminating in an outer end adjacentto the outer end of said outer conductor, a pair of RF output terminalsrespectively coupled to said outer conductor and said inner conductorintermediate the generator RF terminals and said outer ends, meanscoupling the low frequency AC source terminal and the DC source terminalto the outer end of said inner conductor, a first filter elementdisposed in said conductors between said RF output terminals and theouter ends of said conductors and providing a low impedance bypass pathfor the second harmonic of the predetermined frequency, a second filterelement disposed in said conductors between said RF output terminals andthe outer ends of said conductors and providing a low impedance bypasspath for the third harmonic of the predetermined frequency, a thirdfilter element disposed in said conductors between said RF outputterminals and the outer ends of said conductors and including a cavityresonant at the predetermined frequency and presenting a high seriesimpedance to the transmission of the predetermined frequency along saidconductors to the low frequency AC source terminal and the DC sourceterminal, and a fourth filter element disposed in said conductorsbetween said RF output terminals and the outer ends of said conductorsand including a cavity resonant at the fourth harmonic of thepredetermined frequency and presenting a high series impedance to thetransmission of the fourth harmonic of the predetermined frequency alongsaid conductors to the low frequency AC source terminal and the DCsource terminal, whereby propagation of RF energy at thepredetermined-frequency and the second and third and fourth harmonicsthereof from the generator to the outer ends of said conductors ishighly attenuated without interfering with the transmission of the lowfrequency AC and the DC operating potentials between the sources thereofand the generator.

20. A coupling structure for interconnecting a generator for generatingRF energy of a predetermined ultrahigh RF frequency and a source of DCoperating potentials having at least one terminal and a source of lowfrequency AC energy having at least one terminal, wherein said generatorhas an annular outer RF terminal and an inner RF terminal, said couplingstructure comprising a hollow outer conductor coupled to the outer RFterminal of the generator and extending outwardly therefrom andterminating in an outer end, an inner conductor disposed in said outerconductor and coupled to the inner RF terminal of the generator andextending outwardly therefrom and terminating in an outer end adjacentto the outer end of said outer conductor, a pair of RF output terminalsrespectively coupled to said outer conductor and said inner conductorintermediate the generator RF terminals and said outer ends, meanscoupling the low frequency AC source terminal and the DC source terminalto the outer end of said inner conductor, a first cup-like filter memberincluding a cylindrical side wall surrounding the adjacent portion ofsaid inner conductor and concentric therewith and spaced therefrom andan end wall closing said side wall at one end thereof and connected tosaid inner conductor intermediate the ends thereof, said first filtermember being telescopically received in and insulated from said outerconductor, and a second cup-like filter member including a cylindricalside wall surrounding the adjacent portion of said inner conductor andconcentric therewith and spaced therefrom and an end wall closing saidside wall at one end thereof and connected to said inner conductor atthe outer end thereof, the cylindrical side wall of said second filtermember being telescopically received in and insulated from the outer endof said outer conductor, said inner and outer conductors being spacedapart and cooperating to form therebetween a low impedance bypass pathat the second harmonic of the predetermined frequency to effect highattenuation thereof, the cylindrical side wall of said first filtermember cooperating with said outer conductor to form therebetween a lowimpedance bypass path at the third harmonic of the predeterminedfrequency to effect high attenuation thereof, said first filter membercooperating withsaid inner conductor to define a first cavity having aneffective electrical length equal to one-quarter of the wavelength ofthe predetermined frequency and being resonant at the predeterminedfrequency to present a high impedance to the transmission of thepredetermined frequency along said inner conductor, said second filtermember cooperating with said inner conductor to define a second cavityhaving an effective electrical length equal to one-quarter of thewavelength of the fourth harmonic of the predetermined frequency andbeing resonant at the fourth harmonic of the predetermined frequency topresent a high impedance to the transmission of the fourth harmonic ofthe predetermined frequency along said inner conductor, wherebypropagation of RF energy at the predetermined frequency and the secondand third and fourth harmonics thereof from said generator to the outerends of said conductors is highly attenuated without interfering withthe transmission of the low frequency AC and the DC operating potentialsbetween the sources thereof and the generator.

21. The coupling structure set forth in claim 20, wherein the end wallsof said first and second cup-like filter members are both disposedtoward said sources.

22. A coupling structure for interconnecting a generator for generatingRF energy of a predetermined ultra-high RF frequency and a source of DCoperating potentials having at least one terminal and a source of lowfrequency AC operating potential having at least one terminal and a loadfor the RF energy, wherein the generator has an annular outer RFterminal and an inner RF terminal, said coupling structure comprising afirst hollow outer conductor coupled to the outer RF terminal of thegenerator and extending outwardly therefrom and terminating in an outerend, a first inner conductor disposed in said first outer conductor andcoupled to the inner RF terminal of the generator and extendingoutwardly therefrom and terminating at an outer end adjacent to theouter end of said first outer conductor, means coupling the DC sourceterminal and the low frequency AC source terminal to the outer end ofsaid first inner conductor, a second hollow outer conductor coupled tosaid first outer conductor intermediate the generator RF terminals andsaid outer ends and coupled to the load, a second inner conductordisposed in said second outer conductor and coupled to said first innerconductor intermediate the generator RF terminals and said outer endsand coupled to the load, a first filter element disposed in said firstouter conductor between the coupling to said second outer conductor andthe outer ends of said first conductors and providing a low impedancebypass path for the second harmonic of the predetermined frequency, asecond filter element disposed in said first outer conductor betweensaid coupling to said second outer conductor and the outer ends of saidfirst conductors and including a cavity resonant at the predeterminedfrequency and presenting a high series impedance to the transmission ofthe predetermined frequency along said first conductors to the DC sourceterminal and the low frequency AC source terminal, a third filterelement disposed in said first outer conductor between said coupling tosaid second outer conductor and the outer ends of said first conductorsand including a cavity resonant at the fourth harmonic of thepredetermined frequency and presenting a high series impedance to thetransmission of the fourth harmonic of the predetermined frequency alongsaid first conductors to the DC source terminal and the low frequency ACsource terminal, whereby propagation of RF energy at the predeterminedfrequency and the second and fourth harmonics thereof from the generatorto the outer ends of said first conductors is highly attenuated withoutinterfering with the transmission of the DC and the low frequency ACoperating potentials between the sources thereof and the generator, afirst filter section in said second outer conductor and comprisingstructure defining a first cavity having an effective electrical lengthequal to onequarter of the wavelength of the second harmonic of thepredetermined frequency and being resonant at the second harmonic of thepredetermined frequency to present a high impedance to the transmissionof the second harmonic of the predetermined frequency along said secondconductors, and a second filter section in said second outer conductorand comprising structure defining a second cavity having an effectiveelectrical length equal to one-quarter of the wavelength of the fourthharmonic of the predetermined frequency and being resonant at the fourthharmonic of the predetermined frequency to present a high impedance tothe transmission of the fourth harmonic of the predetermined frequencyalong said second conductors, whereby propagation of RF energy at thesecond and fourth harmonics of the predetermined frequency along saidsecond conductors is highly attenuated without interfering with thetransmission of RF energy at the predetermined frequency to the load.

23. A coupling structure for interconnecting a generator for generatingRF energy of a predetermined ultra-high RF frequency and a source of DCoperating potentials having at least one terminal and a source of lowfrequency AC operating potential having at least one terminal and a loadfor the RF energy, wherein the generator has an annular outer RFterminal and an inner RF terminal, said coupling structure comprising afirst hollow outer conductor coupled to the outer RF terminal of thegenerator and extending outwardly therefrom and terminating in an outerend, a first inner conductor disposed in said first outer conductor andcoupled to the inner RF terminal of the generator and extendingoutwardly therefrom and terminating at an outer end adjacent to theouter end of said first outer conductor, means coupling the DC sourceterminal and the low frequency AC source terminal to the outer end ofsaid first inner conductor, a second hollow outer conductor coupled tosaid first outer conductor intermediate the generator RF terminals andsaid outer ends and coupled to the load, a second inner conductordisposed in said second outer conductor and coupled to said first innerconductor intermediate the generator RF terminals and said outer endsand coupled to the load, a first filter element disposed in said firstouter conductor between the coupling to said second outer conductor andthe outer ends of said first conductors and providing a low impedancebypass path for the second harmonic of the predetermined frequency, asecond filter element disposed in said first outer conductor betweensaid coupling to said second outer conductor and the outer ends of saidfirst conductors and including a cavity resonant at the predeterminedfrequency and presenting a high series impedance to the transmission ofthe predetermined frequency along said first conductors to the DC sourceterminal and the loW frequency AC source terminal, a third filterelement disposed in said first outer conductor between said coupling tosaid second outer conductor and the outer ends of said first conductorsand including a cavity resonant at the fourth harmonic of thepredetermined frequency and presenting a high series impedance to thetransmission of the fourth harmonic of the predetermined frequency alongsaid first conductors to the DC source terminal and the low frequency ACsource terminal, a fourth filter element disposed in said first outerconductor between the coupling to said second outer conductor and theouter ends of said first conductors and providing a low impedance bypasspath for the third harmonic of the predetermined frequency, wherebypropagation of RF energy at the predetermined frequency and the secondand third and fourth harmonics thereof from the generator to the outerends of said first conductors is highly attenuated without interferingwith the transmission of the DC and the low frequency AC operatingpotentials between the sources thereof and the generator, a first filtersection in said second outer conductor and comprising structure defininga first cavity having an eifective electrical length equal toone-quarter of the wavelength of the second harmonic of thepredetermined frequency and being resonant at the second harmonic of thepredetermined frequency to present a high impedance to the transmissionof the second harmonic of the predetermined frequency along said secondconductors, a second filter section in said second outer conductor andcomprising structure defining a second cavity having an effectiveelectrical length equal to one-quarter of the wavelength of the fourthharmonic of the predetermined frequency and being resonant at the fourthharmonic of the predetermined frequency to present a high impedance tothe transmission of the fourth harmonic of the predetermined frequencyalong said second conductors, and a third filter section in said secondouter conductor and comprising structure defining a third cavity havingan effective electrical length equal to one-quarter of the wavelength ofthe third harmonic of the predetermined frequency and being resonant atthe third harmonic of the predetermined frequency to present a highimpedance to the transmission of the third harmonic of the predeterminedfrequency along said second conductors, whereby propagation of RF energyat the second and third and fourth harmonics of the predeterminedfrequency along said second conductors is highly attenuated withoutinterfering with the transmissition of RF energy at the predeterminedfrequency to the load.

References Cited UNITED STATES PATENTS 1/ 1955 Devot. 9/1958 Kach.3/1959 Devot. 6/1965 Wantuch. 5/1967 Reker 219-10.55 4/1968 Staats219-10.55 X 11/1968 Schmid 33573 4/1969 Boehm 219-1055 4/1969 Boswell eta1. 21910.55

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION pa 3,536,878 DatedOctober 21, 1970 Inventor(s) Louls Fltzmayer et 1 It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 26, line 10, "3,448,386" should read 3,440,386

Signed and sealed this 13th day of April 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

FORM F'O-105O (10-69) I g A I I h F n w

